1. Field of the Invention
The present invention relates to a shaft-and-yoke coupling structure and a vehicle steering system including the same.
2. Description of Related Arts
Generally, in motor vehicles, a steering shaft joined to a steering wheel (steering member) and a steering gear shaft provided as, for example, a pinion shaft are joined to each other via a so-called intermediate shaft being a steering transmission shaft.
Such an intermediate shaft commonly adopts a telescopic shaft in which an inner shaft and a cylindrical outer shaft are joined to each other in an axially and relatively slidable manner. End portions of the inner and outer shafts of the intermediate shaft are joined, respectively, to the corresponding steering shaft and pinion shaft via a universal joint.
Yokes of such universal joints have conventionally been fixed integrally by welding, respectively, to inner and outer shafts of an intermediate shaft (refer to, for example, paragraph [0011] in Japanese Published Unexamined Utility Model Application No. Hei 5-46649). In some cases, the yokes and the corresponding shafts are fitted in a serrated manner so as to be steerable even in the case of breakage of the welded portions. In addition to the fixation by welding, the shafts are swaged to the corresponding yokes in some cases.
Thus adding a serrated fitting and/or swaging allows a torque to be transmitted between the yokes of the universal joints and the intermediate shaft, even in the case of breakage of the welded portions, and thereby a fail-safe function to be achieved.
However, since the torque transmission between the yokes of the universal joints and the intermediate shaft is maintained sufficiently even in the case of breakage of the welded portions, the driver cannot recognize the occurrence of an abnormality. This may cause the driver to continue driving for a long period of time with no repair.
It is therefore an object of the present invention to provide a shaft-and-yoke coupling structure and a vehicle steering system including the same that can perform a fail-safe function in the case of breakage of a welded portion as well as allowing the occurrence of an abnormality to be recognized by the driver.